Fix cancelled subs bug.

[theLee3]

This PR completes the implementation of the player stream. The following changes are included.

1. [BUG FIX] Set the MethodChannel subscription to null when all listeners are cancelled. Previously just cancelled the sub, which would not allow new subs, as there is a null check before assigning a new sub. Fixes [iOS] GameAuth.isSignedIn not working after user cancel the GameAuth.signIn() #158.

2. [BUG FIX] Set the cached player to null when an error occurs across the player stream.

3. [QoL] Make the player stream distinct, meaning that current listeners do not receive duplicate data when new listeners are added or legacy methods are called.

4. [QoL] Bump iOS deployment target to 12.4 which provides better feedback to the dev when building for iOS, as it is now necessary due to the use of gamePlayerID (playerID is deprecated). It will also be necessary when upgrading from other deprecated GameKit methods.

5. [QoL] Update some information in the podspec file to match the version number and provide links to the GitHub.

[PacifistaBot]

	Fails
🚫	Flutter analyzer failed. Please fix the issues reported by the analyzer. Error: Command failed: flutter analyze 1 issue found. (ran in 6.9s)

	Warnings
⚠️	Please update the CHANGELOG.md file.
⚠️	Please update the documentation in the docs/ folder.

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